1. Field of the Invention
This invention relates to an electro-optical device, particularly a liquid crystal display device using a nematic liquid crystal having positive dielectric anisotropy.
2. Description of the Prior Arts
Nematic liquid crystal (hereinafter abbreviated as N-liquid crystal) display device effects displaying by utilization of changes in optical characteristics by application of electrical field, magnetic field, ultra-sonic wave, etc., and is known to be used for optical modulation and others. This is generally constituted of N-liquid crystal sandwiched between two sheets of substrates, of which one is transparent, provided face to face at an interval of 50.mu. or less, and its changes in molecular arrangement by application of electrical field, magnetic field, ultra-sonic wave, etc. are utilized for optical modulation. In the case of applying an electrical field, an electrode plate having a thin layer of electroconductive coating applied on one surface of a supporting plate such as glass plate is used as the substrate.
Compounds for the above N-liquid crystal are classified into two kinds with respect to the molecular structure and dielectric property. One is a kind in which the molecular axis and the electric dipole are substantially perpendicular to each other (this is called as Nn-liquid crystal), and the other is a kind in which they are substantially parallel to each other (this is called Np-liquid crystal). Nn means an N-liquid crystal having negative dielectric anisotropy, and Np an N-liquid crystal having positive dielectric anisotropy. The Np-liquid crystal electro-optical device generally employed in the prior art has a structure having a Np-liquid crystal sandwiched between confronting two sheets of electrode plates. In this case, the molecular axis of the Np-liquid crystal is parallel to the electrode plates, and the direction of the molecular axis will be aligned in substantially the same direction on the plane parallel to the electrode plate, and the electrode plate, and the electrode plates arranges the molecules so that they may be twisted continuously from each other in the direction perpendicular to the electrode. Such an alignment of molecular axis can be achieved by rubbing the electrode surfaces in one direction with a cloth or a paper and injecting a liquid crystal between the two sheets of electrodes superposed so that the rubbed directions may be crossed over each other. In the vicinity of the electrode surface, the molecular axes of liquid crystal are known to be regularly aligned in the polished direction, while the molecular axes to take the directions continuously twisted within the liquid crystal layer.
However, according to this method, alignment cannot be uniform and will disadvantageously be lost within a short period of time. Also, alignment destruction is known to occur under heating.
As the method for overcoming these drawbacks, it is known to rub the electrode plates in one direction with a surfactant used simultaneously. However, the surfactant may cause deterioration of a liquid crystal, and it may further be decomposed or denatured by electrical field upon continuous application of a voltage, whereby alignment may be destroyed. As another method, it is also known to coat the surface of an electrode plate with a certain kind of polymer such as polyimide or polyamideimide, and subjecting the coated polymer layer to aligning treatment. However, for example, in the case of providing a coating of polyimide on an electrode plate, after application of a polyamide carboxylic acid solution using a polar solvent such as dimethylacetamide on an electrode plate, the polyamide carboxylic acid can be subjected to dehydrating ring closure to form a polyimide film. Such a polyimide film, while it is excellent in heat resistance and aligning controllability for the liquid crystal, is required to be treated at a high treating temperature of 250.degree. C. to 300.degree. C. or above during formation of the film, and the solvent employed is strongly toxic, polar solvent, such as dimethylformamide, dimethylacetamide, dimethyl sulfoxide, N-methylpyrrolidone, etc. and must be carefully handled under the present situation. On the other hand, polyvinyl alcohol is a water-soluble resin, and therefore water can be used as the solvent and has no such toxicity as possessed by the solvents as mentioned above so that is can be easily handled. However, when a film formed of polyvinyl alcohol is used as the alignment film, it has the disadvantage of poor durability.